Various forms of line interface circuit, and various desirable features in line interface circuits, are known. In particular, it is known for example from Rosenbaum U.S. Pat. No. 4,484,032 issued Nov. 20, 1984 and entitled "Active Impedance Transformer Assisted Line Feed Circuit" to provide a line interface circuit in which two amplifiers have their outputs coupled to the tip and ring wires of a two-wire telephone line via feed resistors and primary windings of a very small transformer. In this arrangement, a secondary winding of the transformer and a resistive network coupled to the feed resistors provide for sensing of a.c. and d.c. conditions on the line.
In addition, it is known from Rosch et al. U.S. Pat. No. 4,764,956 issued Aug. 16, 1988 and entitled "Active Impedance Line Feed Circuit" to cancel common mode signals and to provide a substantially constant threshold for ground fault current limiting in a line interface circuit.
With evolution of telephone systems, it is desirable to provide improved line interface circuits which in particular have a bandwidth which is sufficiently great to accommodate ISDN (integrated services digital network) services, for example a signal bandwidth of the order of 200 kHz. At the same time, it is desirable to provide improvements in line interface circuits with respect to such features as their size, cost, versatility, and operation especially in relation to fault conditions, common mode signal rejection, and power consumption and dissipation.
Considered generally, there is a need for a line interface circuit which can be used to operate in conjunction with any arbitrary telephone communications line to provide arbitrary voice and data communications services as may be desired at any particular time, which services can be readily changed under software control from a telephone central office processor without requiring any hardware changes of the line interface circuit.
Such a line interface circuit must be able to terminate, and be matched to, lines of various resistive and complex impedances. For example, two-wire telephone lines in North America have terminating impedances of 600 or 900 .OMEGA., with or without a capacitive component of 2.16 .mu.F. Other terminating impedances are used in other countries, and for digital loop services.
In known active impedance line interface circuits, a desired terminating impedance may be generated using a scaled model of the desired impedance or its inverse. A scaled model of the desired impedance is generated by connecting a network having the same topology as the desired impedance in the feedback path of an amplifier circuit, while the inverse is generated by connecting the network in a feed-forward path of an amplifier circuit.
This known technique has disadvantages in that different circuit topologies are required to generate an impedance and its inverse, so that it is not convenient to generate either of these in a selective manner as is desirable for a single line interface circuit to be used in a wide variety of situations. In addition, components are generally connected between amplifier inputs and outputs, so that they must be floating with respect to ground, making the circuit design more difficult than one in which one terminal of the components is grounded. Furthermore, programmability to generate various impedances is achieved using a number of reference components and multipliers, with the result that all of the reference components must track one another in value.
Thus although a single design of line interface circuit for use with any of a variety of lines requiring widely different terminating impedances is desirable, the prior art falls short of enabling this in a practical manner.
An object of this invention is to provide an improved impedance generator, which is especially suitable for use in an active impedance line interface circuit for a communications line and which facilitates implementation in an integrated circuit form.